Unfortunately the one and only status i can receive is QMediaPlayer::LoadingMedia with QMediaPlayer::StoppedState .
The videoWidget doesn't display anything.

Does QMediaPlayer support mjpeg at all?
Although i do not receive neither QMediaPlayer::InvalidMedia status nor any other error.
Of course i could use QDesktopServices to display the stream with a web browser but i'd prefer using my own video widget...

I am facing the same problem but I get an error when loading the ip camera stream http://../mjpeg.cgi
-->"Error: Failed to load source."

You need to add an error handler in your code in order to get this error message:
connect(mediaPlayer, SIGNAL(error(QMediaPlayer::Error)), this, SLOT(handleError()));

In handleError method you can get the error message by this call -> mediaPlayer->errorString()

You said:
"Of course i could use QDesktopServices to display the stream with a web browser but i’d prefer using my own video widget…"
Could you be more explicit about this solution, please ? I am really interested.

thanks for your reply!
QMediaPlayer has a static member hasSupport. I called it with the appropriate mime type (video/x-motion-jpeg) to determine whether it is supported or not.
The function returned the QMultimedia::MaybeSupported enum which is quite funny...